1. Field of the Invention
The present invention relates to a composite nanofiltration membrane, a process for the production thereof and to the use of same for the separation of low molecular weight compounds.
2. Description of the Prior Art
Nanofiltration is a known separation technique which is situated at the boundary between reverse osmosis and ultrafiltration.
Reverse osmosis employs a typically organic or sometimes inorganic membrane, the mass transfer therethrough being based on diffusion. The fluid to be treated diffuses through the membrane. The molecular weight cut-off of the membrane is generally below 100 daltons.
Ultrafiltration also employs an organic or inorganic membrane whose pore size typically ranges from 5 to 100 nm. The liquid effluent passes through the pores of the membrane according to a convective flux. The molecular weight cutoff of the membrane is generally above 1,000 daltons.
As utilized herein, by the term "nAnofiltration membrane" is intended a membrane having a molecular weight cutoff from about 100 to 1,000 daltons.
With such membranes, retention is low for salts (monovalent anion) and nonionized organic molecules of molecular weight below 100; it is high for organic molecules of molecular weight above 300, as well as for multivalent salts, which, in this latter case, is not desirable in this type of separation.
For such nanofiltration membranes, the molecular weight cutoff corresponds to a molecular size in the nanometer range and, therefore, circumscribes the domain of separation situated between reverse osmosis and ultrafiltration.
Organic, optionally composite, nanofiltration membranes are known to this art. However, they present the disadvantages of being mechanically and thermally fragile and sensitive to chemical attack.
Inorganic membranes, tubular membranes in particular, exhibit high chemical, mechanical and thermal resistance, which is one of the principal reasons for their current development in the field of ultrafiltration. However, no membrane of this type is believed to exist which is completely inorganic and permits nanofiltration separations to be carried out.
A membrane which may be a hyperfiltration membrane comprising a porous support made of inorganic material and a microporous membrane made of organic polymer is described in EP-250,327.
The polymeric microporous membrane is an asymmetric and cellular, namely, alveolar, membrane, deposited within the pores of the inorganic membrane by a process of preparation via phase inversion, comprising a stage for coagulation of the polymer and a final heat treatment.
In order to obtain an asymmetric and cellular membrane, the pores of the macroporous inorganic membrane have a high mean pore radius of between 0.5 and 1 .mu.m according to the examples of this '327 patent, which corresponds to a microfiltration membrane.
Furthermore, the polymer thicknesses indicated in the patent (25 to 200 .mu.m) are incompatible with a nanofiltration process.
The principal disadvantage of such composite membranes is the alveolar cellular structure of the organic membrane which confers thereon a certain fragility, especially under the influence of high pressures given the large pore diameter of the inorganic support substrate employed.
C. Guizard et al, Proc. FICTIM' 89 --Montpellier, France, 3-6 July 1989, p. 75-85 (1990) indicates the possibility of using inorganic polymers, polyphosphazenes among others, for producing hyper- or nanofiltration membranes on a porous inorganic membrane. However, this publication does not indicate the pore diameter of the inorganic substrate and the desired results are not attained, since the saccharose retention reported on page 83 does not exceed 50%. Moreover, this publication does not indicate the polyphosphazene deposition technique.
EP-A 302,928 describes an inorganic ultrafiltration membrane coated with a hydrophilic organic polymer, which is more particularly suitable for protein separation. This membrane permits only ultrafiltration operations to be performed.